Bioprosthetic heart valves have been developed that attempt to mimic the function and performance of a native valve. Flexible leaflets are fabricated from biological tissue such as bovine pericardium. In some bioprosthetic heart valve designs, the biological tissue is sewn onto a relatively rigid frame that supports the leaflets and provides dimensional stability when implanted. Although bioprosthetic heart valves can provide excellent hemodynamic and biomechanical performance in the short term, they are prone to calcification and cusp tears, among other failure modes, requiring reoperation and replacement.
Attempts have been made to use synthetic materials, such as polyurethane, among others, as a substitute for the biological tissue, to provide a more durable flexible leaflet prosthetic heart valve, herein referred to as a synthetic leaflet prosthetic heart valve (SLV). However, synthetic leaflet prosthetic heart valves have not become a valid heart valve replacement option since they suffer premature failure, due to, among other things, suboptimal design and lack of a durable synthetic material.
The leaflet moves under the influence of fluid pressure. In operation, the leaflets open when the upstream fluid pressure exceeds the downstream fluid pressure and close when the downstream fluid pressure exceeds the upstream fluid pressure. The leaflet free edges of the leaflets coapt under the influence of downstream fluid pressure closing the prosthetic heart valve to prevent downstream blood from flowing retrograde through the prosthetic heart valve.
Prosthetic heart valve durability under the repetitive loads of the leaflets opening and closing is dependent, in part, on the load distribution between the leaflet and the frame. Further, substantial load is encountered on the leaflet when in the closed position. Mechanical failure of the leaflet can arise, for example, at the mounting edge, where the flexible leaflet is supported by the relatively rigid frame. The repetitive loads of leaflet opening and closing leads to material failure by fatigue, creep or other mechanism, depending in part on the leaflet material. Mechanical failure at the mounting edge is especially prevalent with synthetic leaflets.
The durability of the valve leaflets is also a function of the character of bending by the leaflet during the opening-closing cycle. Small radius bends, creases and intersecting creases, can produce high stress zones in the leaflet. These high stress zones can cause the formation of holes and tears under repetitive loading.
Prosthetic heart valves may be delivered using surgical or transcatheter techniques. A surgical prosthetic heart valve is implanted into a patient using open-heart surgical techniques. The surgical prosthetic heart valve is usually manufactured to have a fixed diameter as opposed to a transcatheter prosthetic heart valve which is required to attain a range of diameters for access and delivery. The surgical prosthetic heart valve is usually provided with a sewing cuff about a perimeter of the prosthetic heart valve to allow for suturing to the native tissue orifice.
In addition to the prosthetic heart valve durability issues discussed above, the transcatheter prosthetic heart valve must also be able to withstand the handling and deployment stresses associated with being compressed and expanded.
A preferred shape of synthetic prosthetic heart valve leaflets has been described many times, but each is different from the others. The various three-dimensional shapes range from spherical or cylindrical to truncated conical intersections with spheres and an “alpharabola”.
The shape most often described as preferable is modeled after the native human aortic valve. Though nature dictates the optimum shape for the native tissues to form a heart valve, we have discovered this is not true for synthetic materials; accordingly, the design specified in the current disclosure is instead intended to place the synthetic material under a minimized stress condition as compared to those based on copies of the native valve. This is partially accomplished through reduced buckling in the leaflet material.